The use of passive ultra-high frequency (UHF) radio frequency identification (RFID) is increasing rapidly, especially in logistic applications. This is partly due to a globally accepted standard, EPC Class 1 Gen 2 (ISO 18000-6c), and numerous successful pilot cases all around the world. It seems that RFID is finally starting to redeem the promises built during the last decades.
In passive UHF RFID, an interrogator (also: tag reader) transmits a radiating electromagnetic field to power up a transponder (also: tag). The interrogator modulates the carrier to send commands to the transponder. The transponder, in turn, responds to the command by changing its reflectivity to form modulation that can be detected with the receiver of the interrogator. Since the transponders are powered by the radiating electromagnetic field, the read range in passive UHF RFID is typically limited by power delivery to the transponder.
The huge promises of RFID have lured many companies and other parties to develop components for the developing industry. There are several integrated circuit (IC) manufacturers, dozens of inlay manufacturers, and hundreds of converters involved in the RFID industry. This diversity has led to a need to create metrics and measurement methods for verifying that the components conform to the applicable standards, and for comparing components with one another. Both the International Organization for Standardization (ISO) and EPCglobal have been active in this field.
In measurements of UHF frequency RFID tags the reflections caused by the surrounding environment typically cause error due to multi-path, i.e., echoed, propagation of RF waves. Due to these radio echoes the signal is transmitted from the transmitter to the tag and then from the tag to the receiver along a number of routes in addition to the direct route, and the signals interfere with each other at, for example, the locations of the tag and the receiver. When certain quantities, such as activation sensitivity, are measured in a frequency domain, waviness is formed in the frequency graph as interference changes with the frequency. As the mutual location of the tag and the reader changes in the environment, respective frequency graphs can be significantly different. Such a measurement result is more descriptive of the measurement environment than the actual frequency dependency of the sensitivity of the tag.
The most widely used tag performance measurements are based on measuring two things: how much radiated power is needed to activate the tag, and how strong is the response of the tag. These two concepts are typically measured as a function of some other parameters such as carrier frequency, orientation, interference, or the properties of the surrounding materials. The measurements may be performed with various different measurement systems. However, there are some common problems resulting from the nature of measuring in the radiating far field. Firstly, it is relatively easy to calibrate the measurement system up to the ports of a measurement device or the end of a cable. However, it is far more difficult to evaluate the magnitude of the radiating power at the location of the tag. Secondly, in order to perform measurements in the radiating far field without the effects of multi-path propagation, an anechoic chamber is typically needed. Such a chamber is, however, a large investment.
In acoustics, it is common to eliminate echoes of the measurement chamber by using the beginning of the measurement result only until the first echo is detected, whereby the responses caused by echoes are not taken into consideration. Due to the high propagation velocity of radio waves and the slow modulation of the carrier this is not usable for measuring RFID tags in an active state.
One possible method for determining operational adjustments of the reader, the type and tuning frequency of the tag as well as the measurement geometry in a target environment is to perform a number of measurements in the target environment and thus search for a usable configuration by bracketing. Such a trial and error method is, however, slow, and it does not guarantee finding even a nearly optimal reliability of operation.
An example of the measurement of tag properties performed in an anechoic chamber is disclosed in WO 2005/086279. The measurement is performed with two antennae (transmission/reception) and it is based on the way the tag located in the field of these two antennae changes the electromagnetic waves arriving to the receiver from the transmitter. The method can only be used in anechoic environments.
US 2004/0137844 discloses a method for adjusting a radio frequency receiver or a receiver and a transmitter on the basis of external interference level. The interference measurements do not, however, contain information about multi-path propagation and they can not be used for considering the interactions caused by the surrounding environment and the actual measurement process.
A white paper called “The test pyramid: a framework for consistent evaluation of RFID tags from design and manufacture to end use” published by Avery Dennison RFID Division (http://www.rfid.averydennison.com/_media/press/9.pdf) discloses a testing framework for RFID tags. The framework is based on a “test cube” allowing measurements of a tag at a defined distance from an antenna of an RFID reader. This setup allows for accurate measurements of tag properties, provided that the test cube itself is kept well calibrated and that the radio reflections from the surrounding environment do not significantly affect the measurement.